Designed for the GUMSHOE system, but entirely transposable to the system of your choice (although GUMSHOE is the best system for investigative horror I have encountered).
Key elements this product brings to you:

Risk Factors

Stability & Stability sources

Psychic rules

two sources of evil and horror (monsters)

and an adventure

Risk Factors
A risk factor is a motivator seek out the horror/plot of the game. Risk Factors provide roleplaying motivation for the player, and is a tool for the GM to compel the story forward in a believable manner. There are examples which make things a lot clearer than my meager scribblings, but here is my interpretation:
A player's character is "heroic" and a terrible 'thing' has just grabbed an innocent girl in front of the PC. A sensible player might immediately seek a way to avoid encountering the horror knowing it would lead to certain death, but being "heroic" the GM now presents the PC with the delimma of having to chase the entity down the ally in hopes of saving the girl or taking a stability hit as his very core concept of everything he believed himself to be is challenged by his display of "cowardice".

Stability
Similar to Cthuhlu Sanity scores -when you go negative your character starts to fall apart mentally. The mechanic interact well with the rest of the GUMSHOE system, and the entire section adds immersion to the game. Stability hits come from seeing horrible things, sounds practical right? But that means seeing dead bodies, seeing people you know killed, seeing people tortured, all the scenes that cry out "Oh! The Humanity!" - these all call for checks which can result in hits to your Stability.
Stability Sources are a requirement that characters have to declare, and are basically people, activities, or traits that keep the character sane - spending time with a stability source can rejuvenate your Stability, but conversely loosing a Source can permanently lower your Stability score.
To put this into a brief "actual play" example, during an investigation into a mafia family, an undercover cop gets seen talking with his controller (his old mentor and a source of stability in this mixed up world of drugs and crime, keeping him "on the path" and reminding him of the world he needs to return to). The mobs captures the cop and make him watch the brutal torture and subsequent murder of his long time mentor - causing a horrendous Stability check for seeing such a brutal series of things done to someone important to him, failing his check he is sent into shock (he was already "on the edge") and his Stability Maximum is permanently lowered from the loss of his friend and only link to the outside world.

Psychic Rules
I have not been able to road test these yet so I let future reviewers go into more depth regarding their experiences with this.
As with the rest of the product, this section comes with excellent examples of how the rules work and at the same time, how to evoke that creepiness of the unknown and intangible into your game.
There are no "Psychic Attack" powers for the characters, but abilities such as psychometry, and aura readings, can provide addition insights into difficult problems in a believable and non-magical way.

The Bestiary
I wouldn't call them monsters.
There are two unique examples of major horror antagonists, which you can use in your games or as inspiration in how to bring horror, through misery and avarice, to your game. Well written with good stories to help you know how to facilitate such radical concepts as "boss monsters" you can't shoot to death.

The Adventure
An amusing, complicated, and long adventure about a group of workmates who go on a LARP "team building" day. Unique premise which is easily identifiable to gamers whether they have ever done any Live-Action-Role-Playing or not. This adventure provides an going tutorial on how to execute an investigative horror game, and how to create that illusive sense of foreboding and fear within your players.

CONCLUSION
This product sets itself apart from other Horror Rules because of the way everything interacts with the entire GUMSHOE system in a meaningful and immersive manner. The Risk Factors provide believe, character driven motivations for adventuring; Stability Sources provide opportunities for backgrounds that have meaning to the characters and affect them in a very real way; and the Psychic rules bring my first experience with a subtle use of "powers" that enhances creepiness and provides players with alternative avenues of information gathering and action (perfect for real world settings).

I have given this product 5 stars:
4 because it delivers everything it promises; is written to a standard that I, as an ordinary gamer, cannot find fault with;
and the 5th star because it brings truly new and innovative content to the industry. If you want scary and investigative - get this.